In a camera mechanism, when moving a group of traveling lenses for adjusting focus at a predetermined point in order to form an image of an object on an image formation surface of an image sensor or the like, or when having a read/write head of a hard disc device reach a specified track of the disc, a position detector for detecting a position of a movable part is provided in order to accurately position the movable part such as the group of traveling lenses, or a read/write head.
The position detector is grouped into two groups; one is an optical position detector, which is combined with an optical sensor comprising a luminescent element such as an LED (light emitting diode) and a light-intercepting element such as a photo transistor, with an optical scale formed by a transparent part and a shading part interleaved with each other in a fine pitch, and the other one is a magnetic position detector, which is combined with a magnetic sensor such as a magnetoresistive element (MR element) or a Hall element, etc. with a magnetic scale made from a magnetic body polarized in a fine pitch.
In the optical position detector employing a conventional design, an output quantity of light varies responsive to a temperature. Therefore, an adjustment circuit accommodating the detector to a temperature change, or a control system admitting a temperature difference is necessary to be incorporated. In addition, a relative mechanical position between the luminescent element, optical scale, and light intercepting element must be accurately adjusted so that light radiated from the luminescent element may arrive at the light intercepting element through fine patterns of the trasparent part on the light scale. An adjustment mechanism is thus required. As a result, the optical position detector employing a conventional design becomes large in size and expensive in cost.
Regarding the magnetic position detector employing a conventional design, in contrast to the optical sensor, the magnetic sensor such as a Hall element has a better temperature characteristic than that of the optical sensor, therefore, the adjustment circuit for accommodating the detector for a temperature change is not require The relative positioning adjustment is not as complicated as that for the optical sensor, because only a clearance between the magnetic sensor and magnetic scale has to be adjusted so that a strength pattern of magnetic field formed in a sign-wave and polarized by the magnetic scale can be detected by the magnetic sensor. Thus a simple adjustment mechanism can satisfy the requirement. This proves that the magnetic position detector has an advantage of reducing a sensor in size as well as in cost. The magnetic position detector has been thus employed extensively in the linear actuator which controls a positioning of a group of traveling lenses for a focus adjustment of optical equipment such as a camera.
However, in the linear actuator employing the conventional magnetic position detector, the S/N (ratio of signal to noise) of the magnetic sensor lowers due to leakage flux from the driving magnetic circuit because the magnetic sensor is placed near to the driving magnetic circuit. Providing both the driving magnetic circuit and the magnetic sensor with magnetic shields may be a countermeasure against this problem. It is hard to shield the leakage flux entirely, and yet, the shields contribute to a complicated structure.
In order to reduce this harmful influence to the magnetic sensor due to the leakage flux, a device regarding a disc drive device was disclosed in Japanese Patent Office (Laid open No. is H05-62383.) The structure of this disc drive device is as follows. A pair of magnets and a pair of coils are symmetrically disposed respectively as moving means of a head carriage, and a pair of linear motors of which polarity are reciprocal to those of the magnets symmetrically disposed are employed. A magnetic sensor is mounted to the head carriage, which is on a movable element side, and a magnetic scale is mounted to a member of a stator side disposed on a locus of the magnetic sensor. In addition, another magnetic sensor is disposed at the center between a pair of linear motors symmetrically disposed Another structure contrary to the above one can be available, namely, the magnetic scale is disposed on the head carriage of the movable element side, and the magnetic sensor is disposed on the member of the stator side.
According to the above disc drive device, the leakage fluxes produced from the linear motors symmetrically disposed are cancelled with each other at least in one dimension of the three dimensions because the polarity of each magnet which is mounted to each of the linear motors is reversed at the center of the two linear motors. As a result, the leakage fluxes influence the magnetic sensor less, and the influences of the leakage fluxes toward a sensors driven direction (X direction) and a vertical direction (Z direction) are cancelled with each other.
Even in the above structure, the following problems still remain: (1) Since the polarities of the two magnets are reciprocal with each other, a direction of electric current running through the coil must be directed opposite to the counterpart of another coil by the pair of linear motors symmetrically disposed. Two coils are thus essential in the above structure, which increases a power consumption as well as a size of the linear actuator, and also contributes to an more expensive linear actuator. Magnetic fluxes produced by the currents running through the two coils are not cancelled at the position of the magnetic sensor but are doubled and affect the magnetic sensor. Further, with regard to a horizontal direction (Y direction), because the leakage fluxes are not cancelled, a rate of change of the magnetic resistance of the Hall element is changed, thereby lowering sensitivity of the sensor.